In forthcoming evolutions of cellular radio communication systems, such as Long Term Evolution (LTE) and High-Speed Packet Access (HSPA), the maximum throughput, or data rate, will be higher than in previous systems. Higher throughputs typically require larger system channel bandwidths.
LTE and HSPA are sometimes called “third generation” communication systems and are currently being standardized by the Third Generation Partnership Project (3GPP). The LTE specifications can be seen as an evolution of the current wideband code division multiple access (WCDMA) specifications. An IMT advanced communication system uses an internet protocol (IP) multimedia subsystem (IMS) of an LTE, HSPA, or other communication system for IMS multimedia telephony (IMT). In the IMT advanced system (i.e., a “fourth generation” (4G) mobile communication system), bandwidths of 100 megahertz (MHz) and larger are being considered. The 3GPP promulgates the LTE, HSPA, WCDMA, and IMT specifications, and specifications that standardize other kinds of cellular wireless communication systems.
An LTE system uses orthogonal frequency division multiplex (OFDM) as a multiple access technique (called OFDMA) in the downlink (DL) from system nodes to user equipments (UEs). An LTE system has channel bandwidths ranging from about 1.4 MHz to 20 MHz, and supports throughputs of more than 100 megabits per second (Mb/s) on the largest-bandwidth channels. One type of physical channel defined for the LTE downlink is the physical downlink shared channel (PDSCH), which conveys information from higher layers in the LTE protocol stack and to which one or more specific transport channels are mapped. Control information is conveyed by a physical uplink control channel (PUCCH) and by a physical downlink control channel (PDCCH). LTE channels are described in 3GPP Technical Specification (TS) 36.211 V8.4.0, Physical Channels and Modulation (Release 8) (September 2008), among other specifications.
In an OFDMA communication system like LTE, the data stream to be transmitted in the downlink is portioned among a number of narrowband subcarriers that are transmitted in parallel. In general, a resource block (RB) devoted to a particular UE is a particular number of particular subcarriers used for a particular period of time. An RB is made up of resource elements (REs), each of which is a particular subcarrier used for a smaller period of time. Different groups of subcarriers can be used at different times for different users. Because each subcarrier is narrowband, each subcarrier experiences mainly flat fading, which makes it easier for a UE to demodulate each subcarrier.
Like many modern communication systems, DL transmissions in an LTE system are organized into frames of 10 milliseconds (ms) duration, each frame includes twenty successive time slots, and a subframe includes two successive time slots. OFDMA communication systems are described in the literature, for example, U.S. Patent Application Publication No. US 2008/0031368 A1 by B. Lindoff et al.
FIG. 1 depicts a typical cellular communication system 10. Radio network controllers (RNCs) 12, 14 control various radio network functions, including for example radio access bearer setup, diversity handover, etc. In general, each RNC directs calls to and from a UE, such as a mobile station (MS), mobile phone, or other remote terminal, via appropriate base station(s) (BSs), which communicate with each other through DL (or forward) and uplink (UL, or reverse) channels. In FIG. 1, RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26.
Each BS, or eNodeB in LTE vocabulary, serves a geographical area that is divided into one or more cell(s). In FIG. 1, BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26, although a sector or other area served by signals from a BS can also be called a cell. In addition, a BS may use more than one antenna to transmit signals to a UE. The BSs are typically coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. The RNCs 12, 14 are connected with external networks such as the public switched telephone network (PSTN), the internet, etc. through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).
It should be understood that the arrangement of functionalities depicted in FIG. 1 can be modified in LTE and other communication systems. For example, the functionality of the RNCs 12, 14 can be moved to the eNodeBs 22, 24, 26, and other functionalities can be moved to other nodes in the network. It will also be understood that a base station can use multiple transmit antennas to transmit information into a cell/sector/area, and those different transmit antennas can send respective, different pilot signals.
Fast and efficient cell searches and received signal measurements are important for a UE to get and stay connected to a suitable cell, which can be called a “serving cell”. On a regular basis, a UE measures its received signal strength and signal quality of each cell it detects, including its serving cell, to determine whether a new serving cell should be selected. The new cell can be on the same frequency as a current cell or on a different frequency. The UE measures a reference signal received power (RSRP), which can be defined as the average UE-received signal power of reference symbols (RS) transmitted by an eNodeB, on its serving cell as well as on neighboring cells that the UE has detected as a result of a cell search procedure, as specified for example in Section 5.2 of 3GPP TS 36.304 V8.4.0, User Equipment (UE) Procedures in Idle Mode (Release 8) (December 2008). The eNodeBs control the transmit power levels of the UEs by sending them respective control commands, and from time to time, the UEs can send reports of its perceived channel quality and other signal parameters to one or more eNodeBs.
FIG. 2 shows an arrangement of subcarriers in resource blocks in a subframe in an LTE system. The frequency range depicted in FIG. 2 includes twenty-seven subcarriers, only nine of which are explicitly indicated. In FIG. 2, the RBs, which are indicated by dashed lines, each include twelve subcarriers spaced apart by fifteen kilohertz (kHz), which together occupy 180 kHz in frequency and 0.5 ms in time, or one time slot. In an LTE system, an RB spans either twelve subcarriers with a subcarrier bandwidth of 15 kHz or twenty-four subcarriers with a subcarrier bandwidth of 7.5 kHz, each over a slot duration of 0.5 ms. FIG. 2 shows each time slot including seven OFDM symbols, or REs, each of which has a short (normal) cyclic prefix, although six OFDM symbols having long (extended) cyclic prefixes can be used instead in a time slot. It will be understood that RBs can include various numbers of subcarriers for various periods of time.
RS transmitted by a first transmit (TX) antenna of an eNodeB are denoted R and by a possible second TX antenna in the node are denoted by S. In FIG. 2, RS are depicted as transmitted on every sixth subcarrier in OFDM symbol 0 and OFDM symbol 4 (because the symbols have short cyclic prefixes) in every slot. Also in FIG. 2, the RSs in symbols 4 are offset by three subcarriers relative to the RS in OFDM symbol 0, the first OFDM symbol in a slot.
Besides reference signals, predetermined synchronization signals are transmitted in LTE in a hierarchical cell search scheme, similar to WCDMA, in which synchronization acquisition and cell group identifier are obtained from different synchronization channel (SCH) signals. Thus, a primary synchronization channel (P-SCH) signal and a secondary synchronization channel (S-SCH) signal are defined with a pre-defined structure in Section 6.11 of 3GPP TS 36.211. For example, P-SCH and S-SCH signals can be transmitted on particular subcarriers in particular time slots. In an LTE system, the eNodeBs transmit two different synchronization signals: a primary synchronization signal (PSS) and a secondary synchronization signal (SSS). Primary and secondary synchronization signals are described in U.S. Patent Application Publication No. US 2008/0267303 A1 by R. Baldemair et al. FIG. 2 shows the SSS and PSS as OFDM symbols 5, 6 (assuming operation with the short cyclic prefix and frequency-division duplex (FDD). Current LTE systems have the PSS and SSS symbols transmitted in the middle six resource blocks in subframes 0 and 5.
With FDD in a system like LTE, DL and UL data transmissions are scheduled independently of each other, and so at some times there is only DL transmission, at other times there is only UL transmission, and at still other times there are both DL and UL transmissions at the same time. UL-DL frame timing is specified in Section 8 of 3GPP TS 36.211.
Although LTE and other UEs are often said to use one transmit and two receive antennas, it is usually the case today that the UE has only two physical antennas. As a result, the transmit path and one of the receive paths share the same antenna, as shown in FIG. 3, which is a block diagram of a portion of a UE 300 for an FDD system. The UE 300 includes a first antenna 302 and a second antenna 304, a bandpass filter 306, a duplex bandpass filter 308, two low-noise amplifiers (LNAs) 310, 312, and a power amplifier (PA) 314. As depicted, the UE 300 includes a diversity receiver that has two receive paths, a first path that includes the antenna 302, filter 306, and LNA 310, and a second path that includes the antenna 304, duplex filter 308, and LNA 312. The UE 300 also includes a transmit path that includes the PA 314, duplex filter 308, and antenna 304. The minimum requirements for the receiver's reference sensitivity (REFSENS) are specified in Section 7.3 of 3GPP TS 36.101 V8.3.0, E-UTRA UE Radio Transmission and Reception (Release 8) (September 2009), which basically specifies the noise factor of the receiver.
In the UE 300, leakage between the UL (transmit) and the DL (receive) paths when the UE is scheduled for both DL and UL data in the same subframe results in noise components around zero frequency (d.c.) in the spectra of demodulated DL subcarriers. Leakage can arise, for example, from inductive coupling Lcpl between the antennas 302, 304 and from inductive coupling Lrx and resistive coupling atx-rx between the portions of the duplex filter 308. In general, the noise spectrum is symmetric around zero frequency and has a bandwidth that is twice as large as the bandwidth for UL transmission. In effect, leakage raises the noise floor for affected subcarriers, which degrades the overall DL demodulation performance. Such noise components constitute leakage-induced interference with the received signal and are due mainly to second-order non-linearity products.
Second-order non-linearity in the RF or baseband components squares the (modulated) transmit leakage signal, producing d.c. and modulated components around d.c. in the receive bandwidth of a direct-conversion receiver. The modulated part appears with twice the bandwidth because of the squaring (i.e., convolution in the frequency domain). Other effects can also arise, such as self-mixing, where the strong leaking transmit signal couples to the UL mixer's input port and is multiplied with itself, that produce modulated signals of twice the transmit bandwidth around d.c. in the receive bandwidth. The aim of the so-called duplex filter 308 is to prevent noticeable leakage of the transmit signal into at least the in-band portion of the receive path spectrum, which is not easy due to the need for a sensitive receiver.
The scheduling of DL and UL frames, which is currently done in the eNodeB, does not take such UE-specific interference into account, which can lead to UE demodulation performance degradation. Therefore, there is a need for improved methods and apparatus of DL/UL scheduling that are aware of UE-specific interference.